Parallel block allocation for declustered logical disks

ABSTRACT

In a method for allocating space on a logical disk, a computer receives an allocation request to allocate a number of requested logical disk extents. The computer selects one of a first group having an array of logical disk extents and a second group having an array of logical disk extents. The computer selects a group having a number of free logical disk extents that is greater than or equal to the number of requested logical disk extents. The logical disk extents in the array of the first group and in the array of the second group correspond to disk blocks on a logical disk. The logical disk spans one or more physical random access disks. The computer locks the selected group to prevent allocating a logical disk extent other than in response to the allocation request.

TECHNICAL FIELD

The present invention relates generally to a file system and morespecifically to allocating disk blocks in a file system.

BACKGROUND

An application program stores and manipulates data in files and indirectories via a file system. The file system allocates andde-allocates disk blocks in response to application program requests.Data stored in the files and directories are broken down into smallersegments of bytes and mapped to disk blocks. Thus disk blocks areallocated to store data according to service requests made by theapplication program. A block allocation map is used to keep track of thedisk blocks as they are allocated so that the disk blocks may beretrieved by the application program. Thus, the algorithm used by a filesystem to allocate disk blocks affects how efficiently the file systemstores and retrieves data and therefore, how efficiently the file systemis able to service requests made by the application program.

File systems based on a sequential access storage layer, such as IBM'sJournaled File System (JFS), realize significant performance benefitswhen accessing disk blocks that are physically, or logically, contiguouson a disk, despite the overhead required to maintain contiguous diskblocks. For example, because a sequential access file system must accessdisk blocks in a predetermined order, the file system could service arequest for a group of disk blocks more efficiently if those disk blockswere physically, or logically, located next to each other in the filesystem, if the same group of disk blocks were dispersed throughout thedisk in a noncontiguous manner, the file system would require more timeto access the disk blocks since the storage media would need to domultiple seeks over the disk, at the storage layer, to read all of thedisk blocks in the group.

File systems utilizing random disk access, however, do not realize muchin performance gains by maintaining physically or logically contiguousdisk blocks. To the contrary, maintaining contiguous disk blocks by thefile system hinders performance if the underlying storage media is arandom access device. Specifically, random access devices can accessdisk blocks consistently, regardless of the physical or logical locationof disk blocks and regardless of the order in which disk blocks areaccessed. In other words, the amount of time in which a random accessdevice can access disk blocks does not change significantly if the diskblocks are not contiguous. Maintaining contiguous disk blocks,therefore, results in unnecessary overhead for the file system whenallocating and de-allocating disk blocks for its files.

SUMMARY

In a first embodiment of the present invention, there is a method forallocating space on a logical disk. A computer receives an allocationrequest to allocate a number of requested logical disk extents. Thecomputer selects one of a first group having an array of logical diskextents and a second group having an array of logical disk extents. Thecomputer selects a group having a number of free logical disk extentsthat is greater than or equal to the number of requested logical diskextents. The logical disk extents in the array of the first group and inthe array of the second group correspond to disk blocks on a logicaldisk. The logical disk spans one or more physical random access disks.The computer locks the selected group to prevent allocating a logicaldisk extent other than in response to the allocation request. Thecomputer allocates the number of requested logical disk extents in theselected group, starting at a first index in the array of the selectedgroup. The computer moves a first pointer from the first index in thearray of the selected group to a second index in the array of theselected group, based at least in part on the number of requestedlogical disk extents. The computer releases the lock on the selectedgroup to allow allocating a logical disk extent other than in responseto the allocation request.

In a second embodiment of the present invention, there is a computersystem for allocating space on a logical disk. The computer system hasone or more processors, one or more computer-readable memories, one ormore computer-readable tangible storage devices, and programinstructions stored on at least one of the one or more storage devicesfor execution by at least one of the one or more processors via at leastone of the one or more memories. The program instructions includeprogram instructions to receive an allocation request to allocate anumber of requested logical disk extents. The program instructionsinclude program instructions to select one of a first group having anarray of logical disk extents and a second group having an array oflogical disk extents. The program instructions selected a group having anumber of free logical disk extents that is greater than or equal to thenumber of requested logical disk extents. The logical disk extents inthe array of the first group and in the array of the second groupcorrespond to disk blocks on a logical disk. The logical disk spans oneor more physical random access disks. The program instructions includeprogram instructions to lock the selected group to prevent allocating alogical disk extent other than in response to the allocation request.The program instructions include program instructions to allocate thenumber of requested logical disk extents in the selected group, startingat a first index in the array of the selected group. The programinstructions include program instructions to move a first pointer fromthe first index in the array of the selected group to a second index inthe array of the selected group, based at least in part on the number ofrequested logical disk extents. The program instructions include programinstructions to release the lock on the selected group to allowallocating a logical disk extent other than in response to theallocation request.

In a third embodiment of the present invention, there is a computerprogram product for allocating space on a logical disk. The computerprogram product has one or more computer-readable tangible storagedevices and program instructions stored on at least one of the one ormore storage devices. The program instructions include programinstructions to receive an allocation request to allocate a number ofrequested logical disk extents. The program instructions include programinstructions to select one of a first group having an array of logicaldisk extents and a second group having an array of logical disk extents.The program instructions selects a group having a number of free logicaldisk extents that is greater than or equal to the number of requestedlogical disk extents. The logical disk extents in the array of the firstgroup and in the array of the second group correspond to disk blocks ona logical disk. The logical disk spans one or more physical randomaccess disks. The program instructions include program instructions tolock the selected group to prevent allocating a logical disk extentother than in response to the allocation request. The programinstructions include program instructions to allocate the number ofrequested logical disk extents in the selected group, starting at afirst index in the array of the selected group. The program instructionsinclude program instructions to move a first pointer from the firstindex in the array of the selected group to a second index in the arrayof the selected group, based at least in part on the number of requestedlogical disk extents. The program instructions include programinstructions to release the lock on the selected group to allowallocating a logical disk extent other than in response to theallocation request.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a computer system for allocating and de-allocatinglogical disk extents, according to one embodiment of the presentinvention.

FIG. 2 is a flow chart of an allocation program of FIG. 1 for allocatinglogical disk extents, according to one embodiment of the presentinvention.

FIG. 3 is a flow chart of a de-allocation program of FIG. 1 forde-allocating logical disk extents, according to one embodiment of thepresent invention.

FIG. 4 is a state diagram for an allocation group, according to oneembodiment of the present invention.

FIG. 5 illustrates hardware and software components of the computersystem of FIG. 1.

DETAILED DESCRIPTION

The present invention will now be described with reference to thefigures. FIG. 1 illustrates a computer system 100 for allocating andde-allocating logical disk extents, according to one embodiment of thepresent invention. Computer system 100 has a logical disk space 102.Logical disk space 102 is built on, and spans, one or more physicalrandom access disk drives. Logical disk space 102 is a representation ofavailable space in the underlying one or more physical random accessdisk drives. Logical disk space 102 is divided into logical disksegments 104A, 104B, and 104C. Maps 106A-C track logical disk extentallocations within logical disk segments 104A-C, respectively. Althoughthe example embodiment illustrates three logical disk segments 104A-C,it should be understood that a logical disk space may include any numberof logical disk segments.

Logical disk segment 104A is divided into one or more allocation groups107A-C, logical disk segment 104B is divided into one or more allocationgroups 107D-F, and logical disk segment 104C is divided into one or moreallocation groups 107G-1. Maps 106A-C therefore represent an array ofallocation groups. When a new physical disk is added to logical diskspace 102 of computer 100, the new disk space is divided up intoallocation groups. The new allocation groups may be added to the lastlogical disk segment 104C, or the new allocation groups may be added toa new logical disk segment. Maintaining multiple allocation groups107A-I enables computer 100 to service multiple requests for blockallocation in a parallel manner.

Allocation groups 107A-I include an array of fixed size logical diskextents 108A-I, respectively. A logical disk extent is the basic unit ofallocation as defined in maps 106A-C. A logical disk extent is separatefrom an input/output (I/O) block, which is used to interact with akernel's memory manager, such as the virtual memory manager of thekernel of the IBM® AIX® operating system. A logical disk extent is alsoseparate from a physical disk block, which is the basic unit of I/Ounderlying the logical disk space 102. Maintaining separate block sizesfor different layers allows for flexibility in configuring I/O at boththe kernel layer and at the physical disk layer.

Each allocation group 107A-I has a respective allocation pointer 114A-Ifor pointing to the next logical disk extent available for allocationwhen servicing an allocation request. Each allocation group 107A-I hasan allocation lock for locking an allocation group. Setting theallocation lock prevents a logical disk extent from being allocatedtwice in response to two separate and simultaneous allocation requeststhat are each from two separate threads. Each allocation group 107A-Ihas a respective de-allocation pointer 116A-I for pointing to the nextlogical disk extent available for de-allocation when an allocatedlogical disk extent is no longer required and is being released. Eachallocation group 107A-I has a de-allocation lock for locking anallocation group. Setting the de-allocation lock prevents the allocationgroup from receiving a second de-allocation request on a second threadwhile a first de-allocation request from a first thread is beingprocessed.

Computer 100 has an allocation program 110 for allocating logical diskextents as requested by threads, programs, or other services. Computer100 has a de-allocation program 112 for de-allocating logical diskextents that are no longer needed by a requesting thread, program, orservice. In an example embodiment, allocation program 110 andde-allocation program 112 constitute a file system of an operatingsystem. As will be described in more detail with respect to the flowcharts of FIGS. 2 and 3, allocation program 110 and de-allocationprogram 112 allocate logical disk extents and release logical diskextents, respectively, in a round-robin manner. Thus, the overhead ofrearranging logical disk extents, or disk blocks, in order to maintaincontiguity thereof, is eliminated. However, because logical disk space102 is built on one or more random access physical disk drives, the timerequired to allocate and release logical disk extents remains relativelyconstant over time even though the logical disk extents may becomenon-contiguous over time.

FIG. 2 is a flow chart of allocation program 110 of FIG. 1 forallocating logical disk extents, according to one embodiment of thepresent invention. At step 202, allocation program 110 receives anallocation request requesting a number of logical disk extents. At step204, allocation program 110 determines whether one of allocation groups107A-I can satisfy the allocation request, without first setting theallocation lock to lock the allocation group. Without setting theallocation lock before determining whether the allocation group cansatisfy the allocation request, it is possible that application program110 can allocate some or all of the number of free logical disk extentsin response to a second allocation request. Consequently, adetermination that the allocation group can satisfy the originalallocation request may be inaccurate. However, determining whether theallocation group can satisfy the allocation request without firstsetting the allocation lock is advantageous in that it improves theperformance of allocation program 110 and helps reduce queuing ofallocation requests at an allocation group that would occur by settingthe allocation lock.

Allocation program 110 determines whether the allocation group cansatisfy the allocation request by calculating the free number of logicaldisk extents for the allocation group, e.g., allocation group 107A, inone of two ways, depending on whether an allocation pointer, e.g.,allocation pointer 114A, is less than or greater than a de-allocationpointer, e.g., de-allocation pointer 116A. The allocation pointer andthe de-allocation pointer are initially both set to 0, e.g., the firstindex in array of fixed size logical disk extents 108A, but are adjustedby allocation program 110 and de-allocation program 112, as will bedescribed. Since allocation program 110 performs logical disk extentallocation in a round-robin manner, and since the array of fixed sizelogical disk extents is fixed in size, the allocation pointer will, atsome point, reach the end of the array and then return to the beginningof the array as the logical disk extents at the end of the allocationgroup become allocated and the logical disk extents at the beginning ofthe array are released. Thus, if the allocation pointer is greater thenor equal to the de-allocation pointer, allocation program 110 calculatesthe number of free logical disk extents by adding the size of the array(AS) to de-allocation pointer (DP) and then subtracting from that totalallocation pointer (AP).# Of Free Logical Disk Extents=DP+AS−AP

If allocation pointer is less then de-allocation pointer, allocationprogram 110 calculates the number of free logical disk extents bysubtracting allocation pointer (AP) from de-allocation pointer (DP).# Of Free Logical Disk Extents=DP−AP

At step 206, allocation program 110 compares the number of logical diskextents requested in the allocation request to the calculated number offree logical disk extents. If allocation program 110 determines that thenumber of logical disk extents requested in the allocation request isgreater than the number of free logical disk extents (decision 206, nobranch), then allocation program 110 proceeds to another allocationgroup, e.g., allocation group 107B, at step 208, and repeats step 204.In an example embodiment, if allocation program 110 determines that noneof the allocation groups have available the number of logical diskextents requested in the allocation request, allocation program 110breaks down the allocation request into a number of smaller requests.For example, if 8 logical disk extents are requested in the allocationrequest and two different allocation groups each have 4 free logicaldisk extents, allocation program 110 may divide the request for 8logical disk extents into two requests of 4 logical disk extents,respectively.

If allocation program 110 determines that the number of logical diskextents requested in the allocation request is less than or equal to thenumber of free logical disk extents (decision 206, yes branch), thenallocation program 110 sets the allocation lock to lock the allocationgroup at step 210, thereby preventing free logical disk extents of theallocation group from being allocated to a requester of a secondallocation request. In one example, at step 210, allocation program 110may set the allocation lock of a first allocation group to lock thefirst allocation group, and then allocate logical disk extents from asecond allocation group to a second thread in response to receiving asecond allocation request from the second thread while the allocationlock is set. In another example, allocation program 110 may place thesecond allocation request in a queue until the allocation lock isreleased.

At step 212, allocation program 110 again determines whether theallocation group can satisfy the allocation request by calculating thenumber of free logical disk extents for the allocation group, using thesame method described in step 204. Allocation program 110 performs thesame determination a second time, as the allocation lock that locks theallocation group was not set when allocation program 110 performed thedetermination the first time at step 204. In between allocation program100 performing the determination at step 204 and setting the allocationlock to lock the allocation group at step 210, logical disk extentsdetermined to have been initially free at step 204 may have beenallocated in response to a second allocation request. Thus, allocationprogram 110 performs a second check at step 212.

If allocation program 110 determines that the number of logical diskextents requested in the allocation request is now greater than thenumber of free logical disk extents (decision 214, no branch), thenallocation program 110 proceeds to another allocation group, at step208, and repeats step 204.

If allocation program 110 determines that the number of logical diskextents requested in the allocation request is still less than or equalto the number of free logical disk extents (decision 214, yes branch),then allocation program 110 allocates the number of logical disk extentsrequested in the allocation request, at step 216, to service theallocation request. Allocation program 110 allocates a logical diskextent by removing the logical disk extent from a slot in an array oflogical disk extents, e.g., array 108A, of the allocation group, e.g.,allocation group 107A, pointed to by the allocation pointer, e.g.,allocation pointer 114A, and updating a corresponding map, e.g., map106A.

At step 218, allocation program 110 increments the allocation pointer(AP) according to the number of logical disk extents (LDE) allocated instep 216, not to exceed the array size (AS).AP=(AP+LDE) Modulo AS

Since the array is a fixed size, the allocation pointer resets to 0,e.g., the first index in the array, after reaching the last index in thearray, before being further incremented. For example, allocation program110 increments allocation pointer 114A currently pointing to logicaldisk extent #7 in allocation group 107A including 10 logical diskextents to point to logical disk extent #2 after allocation program 110allocates 5 logical disk extents to service a request. At step 220,allocation program 110 releases the allocation lock.

FIG. 3 is a flow chart of a de-allocation program 112 of FIG. 1 forde-allocating logical disk extents, according to one embodiment of thepresent invention. At step 302, de-allocation program 112 receives arequest to de-allocate a number of currently allocated logical diskextents.

At step 304, de-allocation program 112 sets a de-allocation lock, e.g.,de-allocation lock 116A, to lock an allocation group, e.g., allocationgroup 107A, thereby preventing the allocation group from receiving asecond de-allocation request while a first de-allocation request isbeing processed. De-allocation program 112 de-allocates a logical diskextent to the same allocation group from which the logical disk extentwas originally allocated. An address of a logical disk extent isindicative of a corresponding allocation group. Therefore, de-allocationprogram 112 sets a de-allocation lock to lock an allocation group, basedon an address of a logical disk extent to be de-allocated. It should beunderstood that the de-allocation lock does not prevent allocationprogram 110 from allocating logical disk extents. In other words,allocation program 110 and de-allocation program 112 may have locks onthe allocation group simultaneously and may perform allocate andde-allocate procedures, respectively, in parallel.

At step 306, de-allocation program 112 de-allocates the number oflogical disk extents requested to be de-allocated. De-allocation program112 de-allocates a logical disk extent by inserting the logical diskextent back into the array of disk extents, e.g., array 108A, in theallocation group into a slot pointed to by de-allocation pointer 116 andupdating a corresponding map, e.g., map 106A.

At step 308, de-allocation program 112 increments de-allocation pointer(DP), e.g., de-allocation pointer 116A, by the number of logical diskextents (LDE) that were de-allocated in step 306, not to exceed thearray size (AS).DP=(DP+LDE) Modulo AS

At step 310, de-allocation program 112 releases the de-allocation lockon the allocation group.

FIG. 4 illustrates a state diagram for an allocation group 402,according to one embodiment of the present invention. At Time T0 400,allocation pointer 404 and de-allocation pointer 406 are bothinitialized to point to logical disk extent 408A and all logical diskextents 408A-J are available for allocation.

At time T1 408, after allocation program 110 has allocated 7 logicaldisk extents, and after de-allocation program 112 has de-allocated 4logical disk extents, allocation group 402 has 7 logical disk extentsavailable for allocation. Specifically, allocation pointer 404 ispointing to logical disk extent 408H, and de-allocation pointer 406 ispointing to logical disk extent 408E. Thus, logical disk extents 408E-Gare allocated, and logical disk extents 408A-D and logical disk extents408H-J are available for allocation.

At time T2 410, after allocation program 110 has allocated 4 logicaldisk extents, and after de-allocation program 112 has de-allocated 3logical disk extents, allocation group 402 has 6 logical disk extentsavailable for allocation. Specifically, allocation pointer 404 ispointing to logical disk extent 408B, and de-allocation pointer 406 ispointing to logical disk extent 408H. Thus, logical disk extents 408Aand 408H-J are allocated, while logical disk extents 408B-408G areavailable for allocation.

At time T3 412, after allocation program 110 has allocated 5 logicaldisk extents, and after de-allocation program 112 has de-allocated 9logical disk extents, allocation pointer 404 and de-allocation pointer406 are both pointing to logical disk extent 408G. Thus, all 10 logicaldisk extents 408A-J of allocation program 402 are again available forallocation.

FIG. 5 illustrates hardware and software components of computer system100 of FIG. 1. Computer system 100 includes internal components 800 andexternal components 900. Internal components 800 include one or moreprocessors 820, one or more computer-readable RAMs 822 and one or morecomputer-readable ROMs 824 on one or more buses 826, and one or moreguest operating systems 828 and one or more computer-readable tangiblestorage devices 830. The one or more operating systems 828, allocationprogram 110, and de-allocation program 112 are stored on at least one ormore of the computer-readable tangible storage devices 830 for executionby at least one or more of the processors 820 via at least one or moreof the RAMs 822 (which typically include cache memory). In theembodiment illustrated in FIG. 5, each of the computer-readable tangiblestorage devices 830 is a magnetic disk storage device of an internalhard drive. Alternatively, each of the computer-readable tangiblestorage devices 830 is a semiconductor storage device such as ROM 824,EPROM, flash memory or any other computer-readable tangible storagedevice that can store a computer program and digital information. Asused herein, a computer-readable tangible storage device is not to beconstrued as being a transitory signal per se, such as radio waves orother freely propagating electromagnetic waves, electromagnetic wavespropagating through a waveguide or other transmission media (e.g., lightpulses passing through a fiber-optic cable), or electrical signalstransmitted through a wire.

Internal components 800 also include a R/W drive or interface 832 toread from and write to one or more portable computer-readable tangiblestorage devices 936 such as a CD-ROM, DVD, memory stick, magnetic tape,magnetic disk, optical disk or semiconductor storage device. Allocationprogram 110 and De-allocation program 112 can be stored on at least oneor more of the portable computer-readable tangible storage devices 936,read via R/W drive or interface 832 and loaded into at least one of oneor more computer-readable tangible storage devices 830.

Internal components 800 also include a network adapter or interface 836such as a TCP/IP adapter card. Allocation program 110 and De-allocationprogram 112 can be downloaded to computer system 100 from an externalcomputer via a network (for example, the Internet, a local area networkor other, wide area network) and network adapter or interface 836. Fromthe network adapter or interface 836, allocation program 110 andde-allocation program 112 are loaded into at least one of one or morecomputer-readable tangible storage devices 830. The network may comprisecopper wires, optical fibers, wireless transmission, routers, firewalls,switches, gateway computers and/or edge servers.

External components 900 include a computer display monitor 920, akeyboard 930, and a computer mouse 934. Internal components 800 alsoinclude device drivers 840 to interface to computer display monitor 920,keyboard 930 and computer mouse 934. The device drivers 840, R/W driveor interface 832 and network adapter or interface 836 comprise hardwareand software (stored in one or more computer-readable tangible storagedevices 830 and/or one or more computer-readable ROMs 824).

Allocation program 110 and de-allocation program 112 can be written invarious programming languages including low-level, high-level,object-oriented or non object-oriented languages. Alternatively, thefunctions of allocation program 110 and de-allocation program 112 can beimplemented in whole or in part by computer circuits and other hardware(not shown).

The description above has been presented for illustration purposes only.It is not intended to be an exhaustive description of the possibleembodiments. One of ordinary skill in the art will understand that othercombinations and embodiments are possible.

What is claimed is:
 1. A method for allocating space on a logical disk,the method comprising the steps of: a computer receiving an allocationrequest to allocate a number of requested logical disk extents; thecomputer selecting one of a first group comprising an array of logicaldisk extents and a second group comprising an array of logical diskextents, the selected group having a number of free logical disk extentsthat is greater than or equal to the number of requested logical diskextents, the logical disk extents in the array of the first group and inthe array of the second group corresponding to disk blocks on a logicaldisk, the logical disk spanning one or more physical random accessdisks; the computer locking the selected group to prevent allocating alogical disk extent other than in response to the allocation request;the computer allocating the number of requested logical disk extents inthe selected group, starting at a first index in the array of theselected group; the computer moving a first pointer from the first indexin the array of the selected group to a second index in the array of theselected group based at least in part on the number of requested logicaldisk extents; the computer receiving a de-allocation request tode-allocate a second number of allocated logical disk extents of theselected group; subsequent to the step of locking the selected group toprevent allocating a logical disk extent other than in response to theallocation request, and prior to releasing the lock, the computerde-allocating the second number of allocated logical disk extentsrequested for de-allocation; and the computer releasing the lock on theselected group to allow allocating a logical disk extent other than inresponse to the allocation request.
 2. The method of claim 1, furthercomprising the steps of: after the step of the computer locking theselected group and before the step of the computer releasing the lock onthe selected group, the computer receiving a second allocation requestto allocate a second number of requested logical disk extents; thecomputer identifying that the selected group is locked and selectinganother one of the first and second groups that is not locked, theselected other group having a number of free logical disk extentsgreater than or equal to the second number of requested logical diskextents; the computer locking the selected other group to preventallocating a logical disk extent other than in response to the secondallocation request; the computer allocating the second number ofrequested logical disk extents in the selected other group, starting ata first index in the array of the selected other group; the computermoving a second pointer from the first index in the array of theselected other group to a second index in the array of the selectedother group based at least in part on the second number of requestedlogical disk extents; and the computer releasing the lock on theselected other group to allow allocating a logical disk extent otherthan in response to the second allocation request.
 3. The method ofclaim 1, further comprising the steps of: subsequent to the computerreceiving the de-allocation request to de-allocate the second number ofallocated logical disk extents of the selected group; the computerlocking the selected group to prevent de-allocating a logical diskextent other than in response to the de-allocation request; the computerde-allocating the number of allocated logical disks extents in theselected group, starting at an index in the array of the selected groupcorresponding to a second pointer; the computer moving the secondpointer from the index corresponding to the second pointer to theanother index in the array of the selected group based at least in parton the number of allocated logical disk extents to de-allocate; and thecomputer unlocking the selected group to allow de-allocating a logicaldisk extent other than in response to the de-allocation request.
 4. Themethod of claim 1, further comprising the step of: the computercalculating the number of free logical disk extents, wherein: (a) if thesecond index, corresponding to the first pointer, in the array of theselected group is greater than or equal to a third index correspondingto the a second pointer indicative of a starting index for de-allocationof logical disk extents in the array of the selected group, thecalculated number of free logical disk extents is equal to the sum ofthe third index in the array of the selected group and a size of thearray in the selected group, minus the second index in the array of theselected group, and (b) if the second index in the array of the selectedgroup is less than the third index in the array of the selected group,the calculated number of free logical disk extents is equal to the thirdindex in the array of the selected group minus the second index in thearray of the selected group.
 5. The method of claim 1, furthercomprising the step of: before the selecting step, the computerdetermining whether one of the first group and the second group has asufficient number of free logical disk extents to satisfy the receivedallocation request, while free logical disk extents in the array of thefirst group and the array of the second group are available forallocation in response to a second allocation request.
 6. The method ofclaim 1, further comprising the step of: after the locking step, andbefore the allocating step, the computer re-determining whether the oneof the first group and the second group has a sufficient number of freelogical disk extents to satisfy the received allocation request.
 7. Acomputer program product for allocating space on a logical disk, thecomputer program product comprising: one or more hardware computerreadable tangible storage devices; program instructions stored on theone or more computer readable tangible storage devices, the programinstructions comprising: program instructions to receive an allocationrequest to allocate a number of requested logical disk extents; programinstructions to select one of a first group comprising an array oflogical disk extents and a second group comprising an array of logicaldisk extents, the selected group having a number of free logical diskextents that is greater than or equal to the number of requested logicaldisk extents, the logical disk extents in the array of the first groupand in the array of the second group corresponding to disk blocks on alogical disk, the logical disk spanning one or more physical randomaccess disks; program instructions to lock the selected group to preventallocating a logical disk extent other than in response to theallocation request; program instructions to allocate the number ofrequested logical disk extents in the selected group, starting at afirst index in the array of the selected group; program instructions tomove a first pointer from the first index in the array of the selectedgroup to a second index in the array of the selected group based atleast in part on the number of requested logical disk extents; programinstructions to receive a de-allocation request to de-allocate a secondnumber of allocated logical disk extents of the selected group; programinstructions to, subsequent to locking the selected group to preventallocating a logical disk extent other than in response to theallocation request, and prior to releasing the lock, de-allocate thesecond number of allocated logical disk extents requested forde-allocation; and program instructions to release the lock on theselected group to allow allocating a logical disk extent other than inresponse to the allocation request.
 8. The computer program product ofclaim 7, further comprising program instruction, stored on the one ormore computer readable tangible storage devices, to: receive a secondallocation request to allocate a second number of requested logical diskextents; identify that the selected group is locked and select anotherone of the first and second groups that is not locked, the selectedother group having a number of free logical disk extents greater than orequal to the second number of requested logical disk extents; lock theselected other group to prevent allocating a logical disk extent otherthan in response to the second allocation request; allocate the secondnumber of requested logical disk extents in the selected other group,starting at a first index in the array of the selected other group; movea second pointer from the first index in the array of the selected othergroup to a second index in the array of the selected other group basedat least in part on the second number of requested logical disk extents;and release the lock on the selected other group to allow allocating alogical disk extent other than in response to the second allocationrequest.
 9. The computer program product of claim 7, further comprisingprogram instructions, stored on the one or more computer readabletangible storage devices, to: subsequent to receiving the de-allocationrequest, lock the selected group to prevent de-allocating a logical diskextent other than in response to the de-allocation request; de-allocatethe number of allocated logical disks extents in the selected group,starting at an index in the array of the selected group corresponding toa second pointer; move the second pointer from the index correspondingto the second pointer to the another index in the array of the selectedgroup based at least in part on the number of allocated logical diskextents to de-allocate; and unlock the selected group to allowde-allocating a logical disk extent other than in response to thede-allocation request.
 10. The computer program product of claim 7,further comprising program instructions, stored on the one or morecomputer readable tangible storage devices, to: calculate the number offree logical disk extents, wherein: (a) if the second index,corresponding to the first pointer, in the array of the selected groupis greater than or equal to a third index corresponding to the a secondpointer indicative of a starting index for de-allocation of logical diskextents in the array of the selected group, the calculated number offree logical disk extents is equal to the sum of the third index in thearray of the selected group and a size of the array in the selectedgroup, minus the second index in the array of the selected group, and(b) if the second index in the array of the selected group is less thanthe third index in the array of the selected group, the calculatednumber of free logical disk extents is equal to the third index in thearray of the selected group minus the second index in the array of theselected group.
 11. The computer program product of claim 7, furthercomprising program instructions, stored on the one or more computerreadable tangible storage devices, to: before selecting one of the firstand second groups, determine whether one of the first group and thesecond group has a sufficient number of free logical disk extents tosatisfy the received allocation request, while free logical disk extentsin the array of the first group and the array of the second group areavailable for allocation in response to a second allocation request. 12.The computer program product of claim 11, further comprising programinstructions, stored on the one or more computer readable tangiblestorage devices, to: after locking the selected group, and beforeallocating the number of requested logical disk extents in the selectedgroup, re-determine whether the one of the first group and the secondgroup has a sufficient number of free logical disk extents to satisfythe received allocation request.
 13. A computer system for allocatingspace on a logical disk, the computer system comprising: one or morecomputer processors; one or more hardware computer readable tangiblestorage devices; and program instructions stored on at least one of theone or more computer readable tangible storage devices for execution byat least one of the one or more computer processors, the programinstructions comprising: program instructions to receive an allocationrequest to allocate a number of requested logical disk extents; programinstructions to select one of a first group comprising an array oflogical disk extents and a second group comprising an array of logicaldisk extents, the selected group having a number of free logical diskextents that is greater than or equal to the number of requested logicaldisk extents, the logical disk extents in the array of the first groupand in the array of the second group corresponding to disk blocks on alogical disk, the logical disk spanning one or more physical randomaccess disks; program instructions to lock the selected group to preventallocating a logical disk extent other than in response to theallocation request; program instructions to allocate the number ofrequested logical disk extents in the selected group, starting at afirst index in the array of the selected group; program instructions tomove a first pointer from the first index in the array of the selectedgroup to a second index in the array of the selected group based atleast in part on the number of requested logical disk extents; programinstructions to receive a de-allocation request to de-allocate a secondnumber of allocated logical disk extents of the selected group; programinstructions to, subsequent to locking the selected group to preventallocating a logical disk extent other than in response to theallocation request, and prior to releasing the lock, de-allocate thesecond number of allocated logical disk extents requested forde-allocation; and program instructions to release the lock on theselected group to allow allocating a logical disk extent other than inresponse to the allocation request.
 14. The computer system of claim 13,further comprising program instructions, stored on at least one of theone or more computer readable tangible storage devices for execution byat least one of the one or more computer processors, to: receive asecond allocation request to allocate a second number of requestedlogical disk extents; identify that the selected group is locked andselect another one of the first and second groups that is not locked,the selected other group having a number of free logical disk extentsgreater than or equal to the second number of requested logical diskextents; lock the selected other group to prevent allocating a logicaldisk extent other than in response to the second allocation request;allocate the second number of requested logical disk extents in theselected other group, starting at a first index in the array of theselected other group; move a second pointer from the first index in thearray of the selected other group to a second index in the array of theselected other group based at least in part on the second number ofrequested logical disk extents; and release the lock on the selectedother group to allow allocating a logical disk extent other than inresponse to the second allocation request.
 15. The computer system ofclaim 13, further comprising program instructions, stored on at leastone of the one or more computer readable tangible storage devices forexecution by at least one of the one or more computer processors, to:subsequent to receiving the de-allocation request, lock the selectedgroup to prevent de-allocating a logical disk extent other than inresponse to the de-allocation request; de-allocate the number ofallocated logical disks extents in the selected group, starting at anindex in the array of the selected group corresponding to a secondpointer; move the second pointer from the index corresponding to thesecond pointer to the another index in the array of the selected groupbased at least in part on the number of allocated logical disk extentsto de-allocate; and unlock the selected group to allow de-allocating alogical disk extent other than in response to the de-allocation request.16. The computer system of claim 13, further comprising programinstructions, stored on at least one of the one or more computerreadable tangible storage devices for execution by at least one of theone or more computer processors, to: calculate the number of freelogical disk extents, wherein: (a) if the second index, corresponding tothe first pointer, in the array of the selected group is greater than orequal to a third index corresponding to the a second pointer indicativeof a starting index for de-allocation of logical disk extents in thearray of the selected group, the calculated number of free logical diskextents is equal to the sum of the third index in the array of theselected group and a size of the array in the selected group, minus thesecond index in the array of the selected group, and (b) if the secondindex in the array of the selected group is less than the third index inthe array of the selected group, the calculated number of free logicaldisk extents is equal to the third index in the array of the selectedgroup minus the second index in the array of the selected group.
 17. Thecomputer system of claim 13, further comprising program instructions,stored on at least one of the one or more computer readable tangiblestorage devices for execution by at least one of the one or morecomputer processors, to: before selecting one of the first and secondgroups, determine whether one of the first group and the second grouphas a sufficient number of free logical disk extents to satisfy thereceived allocation request, while free logical disk extents in thearray of the first group and the array of the second group are availablefor allocation in response to a second allocation request.
 18. Thecomputer system of claim 17, further comprising program instructions,stored on at least one of the one or more computer readable tangiblestorage devices for execution by at least one of the one or morecomputer processors, to: after locking the selected group, and beforeallocating the number of requested logical disk extents in the selectedgroup, re-determine whether the one of the first group and the secondgroup has a sufficient number of free logical disk extents to satisfythe received allocation request.